Molecular Probes proposes to continue its development of new fluorescent probes in the following areas: FLUORESCEIN SUBSTITUTES. Particularly sought are more photostable derivatives with lower pH sensitivity. DYES WITH LARGE STOKES SHIFTS. Dyes that emit at> 60 nm longer wavelength than their excitation are sought for multi-color fluorescence applications. pH AND METAL ION INDICATORS. We will continue to develop fluorescent intracellular indicators for pH and ions. SITE-SELECTIVE PROBES. Major emphasis will be on site selective probes for single cell imaging, flow cytometry and histochemistry. "CAGED DERIVATIVES." Materials with photolabile protecting groups will be developed for single cell studies. POLAR TRACERS AND CELL VIABILITY PROBES. Emphasis will be on dyes which will be retained by cells through several cell divisions, on dyes that will be enzymatically created within cells, and on polar tracers to follow cell-cell communication. REACTIVE LONG-WAVELENGTH FLUOROPHORES. Research will be on reactive dyes that can be excited by the helium-neon laser or light emitting diodes Products derived from this research will be used to support several collaborations and will be made commercially available.